1. Technical Field
The present disclosure relates to a system of controlling an induction electric motor, and more particularly, to a system of controlling an induction electric motor for controlling an induction electric motor in a high speed region.
2. Description of the Related Art
Capability of operating an electric motor at high speed is required in various industrial fields including the field of electric motors such as fans, pumps, blowers, electric automobiles, electric rail cars and the like.
Among electric motors in various fields, in order to operate the induction electric motor in a high speed region above a preset rated speed, flux weakening control, i.e., operating the electric motor by reducing magnetic flux of a rotor of the electric motor is necessary.
Typically, output voltage of the induction electric motor increases with the speed of the induction electric motor.
The flux weakening control is a technique capable of controlling the electric motor in a high speed region by appropriately regulating output voltage (i.e., counter electromotive force) that increases with the speed of electric motor and then controlling the electric motor, and in turn securing voltage margin that can be synthesized in an inverter.
More specifically, the flux weakening control means a method of reducing magnetic flux of the rotor when an output voltage value is above a preset rated voltage.
In case of AC electric motors among induction electric motors, at the time of vector control (or filed oriented control) based on magnetic flux of the rotor using a flux controller, counter electromotive force is restricted by reducing electric current of magnetic flux component in flux weakening operation range.
This flux weakening control based on control of electric current has problems in that its structure is complicated and dynamic characteristics vary depending on selection of gain of the flux controller.
FIG. 1 is a block diagram showing configuration of a system of controlling an induction electric motor according to the prior art.
Referring to FIG. 1, three phase AC power which is output from an AC power supply 102 is converted into DC power through a diode 103, and the converted DC power is filtered through a filter and then applied to a PWM inverter 104. The DC power applied to the PWM inverter 104 is converted into AC power depending on a gating signal which is output from a magnetic flux controller 106 and a vector control system 107, and then it is delivered to an induction electric motor 105.
FIG. 2 is a diagram showing a magnetic flux controller for generating magnetic flux in the system of controlling the induction electric motor according to the prior art.
When vector control is performed by the magnetic flux controller 106 in FIG. 2, equation for d-axis magnetic flux of the rotor in the coordinate system rotating at synchronous speed is as follows:
                                          ⅆ                          λ              dre                                            ⅆ            t                          =                                            R              r                        ⁢                                          L                m                                            L                r                                      ⁢                          i              dse                                -                                                    R                r                                            L                r                                      ⁢                          λ              dre                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Equation 1 is an equation for d-axis magnetic flux of the rotor in the coordinate system rotating at synchronous speed when vector control is performed. In Equation 1, λdre indicates d-axis magnetic flux of the rotor, Lm indicates magnetizing inductance, Lr indicates inductance of the rotor, and Rr indicates resistance of the rotor.
Referring to FIG. 2, the magnetic flux controller 106 receives d-axis magnetic flux command, λdre* and magnetic flux, λdre of the synchronous coordinate system. A current control unit 203 performs a proportional integral according to formula shown in FIG. 2 by using the received d-axis magnetic flux command, λdre* and the magnetic flux, λdre. Feed forward unit 202 feeds forward a change portion of the magnetic flux λdre according to formula shown in FIG. 2.
Command current, idse* generated by the current control unit 203 and the feed forward unit 202 is converted to magnetic flux λdre through proportional integral by means of a magnetic flux generating unit 204. The magnetic flux converted by the magnetic flux generating unit 204 is delivered to the vector control system 107.
In FIG. 2, “S” stands for Laplace operator, that is, a differential operation. In other words, the current control unit 203 performs integral operation denoted by “1/S” by using a proportional gain, Kp and an integral gain, Ki. Then, the magnetic flux generating unit 204 performs a differential operation indicated as “S”.
The vector control system 107 generates a gating signal based on the converted magnetic flux, λdre and applies it to the PWM inverter 104.
As such, the conventional magnetic flux controller 106 has problems in that since it includes a differential operation and an integral operation, which are relatively complex operation process, there is a limitation to improve performance of operation speed due to such complex and excessive operation. In particular, since the existing magnetic flux controller 106 performs an integral operation by using a proportional gain, Kp and an integral gain, Ki, there is a problem that the performance fluctuates greatly depending on the proportional gain, Kp and integral gain, Ki.
That is, there is a problem that the larger the proportional gain, Kp and the integral gain, Ki, the faster the operation speed due to reduction of the number of operations, but the larger the error.
On the other hand, there is also a problem that the smaller the proportional gain, Kp and the integral gain, Ki, the smaller the error, but the slower the operation speed due to increase of the number of operations.
Further, it is possibly not to prevent increase in output voltage of the electric motor properly due to error of the magnetic flux controller 106, or otherwise it is possibly for the gating signal not to be supplied properly due to slow operation speed of the magnetic flux controller 106. As a result, there is a problem that control on the induction electric motor 105 by a system becomes unstable depending on the gains Kp and Ki of the magnetic flux controller 106.